Bad Column Management with Bit Information in Non-Volatile Memory Systems

ABSTRACT

Column based defect management techniques are presented. Each column of the memory has an associated isolation latch or register whose value indicates whether the column is defective, but in addition to this information, for columns marked as defective, additional information is used to indicate whether the column as a whole is to be treated as defective, or whether just individual bits of the column are defective. The defective elements can then be re-mapped to a redundant element at either the appropriate bit or column level based on the data. When a column is bad, but only on the bit level, the good bits can still be used for data, although this may be done at a penalty of under programming for some bits, as is described further below. A self contained Built In Self Test (BIST) flow constructed to collect the bit information through a set of column tests is also described. Based on this information, the bad bits can be extracted and re-grouped into bytes by the controller or on the memory to more efficiently use the column redundancy area.

FIELD OF THE INVENTION

This invention relates generally to non-volatile semiconductor memorysuch as electrically erasable programmable read-only memory (EEPROM) andflash EEPROM and, more specifically, to techniques for handling defectsin such memories.

BACKGROUND OF THE INVENTION

Solid-state memory capable of nonvolatile storage of charge,particularly in the form of EEPROM and flash BEPROM packaged as a smallform factor card, has recently become the storage of choice in a varietyof mobile and handheld devices, notably information appliances andconsumer electronics products. Unlike RAM (random access memory) that isalso solid-state memory, flash memory is non-volatile, retaining itsstored data even after power is turned off. In spite of the higher cost,flash memory is increasingly being used in mass storage applications.Conventional mass storage, based on rotating magnetic medium such ashard drives and floppy disks, is unsuitable for the mobile and handheldenvironment. This is because disk drives tend to be bulky, are prone tomechanical failure and have high latency and high power requirements.These undesirable attributes make disk-based storage impractical in mostmobile and portable applications. On the other hand, flash memory, bothembedded and in the form of a removable card is ideally suited in themobile and handheld environment because of its small size, low powerconsumption, high speed and high reliability features.

EEPROM and electrically programmable read-only memory (EPROM) arenon-volatile memory that can be erased and have new data written or“programmed” into their memory cells. Both utilize a floating(unconnected) conductive gate, in a field effect transistor structure,positioned over a channel region in a semiconductor substrate, betweensource and drain regions. A control gate is then provided over thefloating gate. The threshold voltage characteristic of the transistor iscontrolled by the amount of charge that is retained on the floatinggate. That is, for a given level of charge on the floating gate, thereis a corresponding voltage (threshold) that must be applied to thecontrol gate before the transistor is turned “on” to permit conductionbetween its source and drain regions.

The floating gate can hold a range of charges and therefore can beprogrammed to any threshold voltage level within a threshold voltagewindow. The size of the threshold voltage window is delimited by theminimum and maximum threshold levels of the device, which in turncorrespond to the range of the charges that can be programmed onto thefloating gate. The threshold window generally depends on the memorydevice's characteristics, operating conditions and history. Eachdistinct, resolvable threshold voltage level range within the windowmay, in principle, be used to designate a definite memory state of thecell.

The transistor serving as a memory cell is typically programmed to a“programmed” state by one of two mechanisms. In “hot electroninjection,” a high voltage applied to the drain accelerates electronsacross the substrate channel region. At the same time a high voltageapplied to the control gate pulls the hot electrons through a thin gatedielectric onto the floating gate. In “tunneling injection,” a highvoltage is applied to the control gate relative to the substrate. Inthis way, electrons are pulled from the substrate to the interveningfloating gate.

The memory device may be erased by a number of mechanisms. For EPROM,the memory is bulk erasable by removing the charge from the floatinggate by ultraviolet radiation. For EEPROM, a memory cell is electricallyerasable, by applying a high voltage to the substrate relative to thecontrol gate so as to induce electrons in the floating gate to tunnelthrough a thin oxide to the substrate channel region (i.e.,Fowler-Nordheim tunneling.) Typically, the EEPROM is erasable byte bybyte. For flash EEPROM, the memory is electrically erasable either allat once or one or more blocks at a time, where a block may consist of512 bytes or more of memory.

Examples of Non-Volatile Memory Cells

The memory devices typically comprise one or more memory chips that maybe mounted on a card. Each memory chip comprises an array of memorycells supported by peripheral circuits such as decoders and erase, writeand read circuits. The more sophisticated memory devices also come witha controller that performs intelligent and higher level memoryoperations and interfacing. There are many commercially successfulnon-volatile solid-state memory devices being used today. These memorydevices may employ different types of memory cells, each type having oneor more charge storage element.

FIGS. 1A-1E illustrate schematically different examples of non-volatilememory cells.

FIG. 1A illustrates schematically a non-volatile memory in the form ofan EEPROM cell with a floating gate for storing charge. An electricallyerasable and programmable read-only memory (EEPROM) has a similarstructure to EPROM, but additionally provides a mechanism for loadingand removing charge electrically from its floating gate upon applicationof proper voltages without the need for exposure to UV radiation.Examples of such cells and methods of manufacturing them are given inU.S. Pat. No. 5,595,924.

FIG. 1B illustrates schematically a flash EEPROM cell having both aselect gate and a control or steering gate. The memory cell 10 has a“split-channel” 12 between source 14 and drain 16 diffusions. A cell isformed effectively with two transistors T1 and T2 in series. T1 servesas a memory transistor having a floating gate 20 and a control gate 30.The floating gate is capable of storing a selectable amount of charge.The amount of current that can flow through the T1's portion of thechannel depends on the voltage on the control gate 30 and the amount ofcharge residing on the intervening floating gate 20. T2 serves as aselect transistor having a select gate 40. When T2 is turned on by avoltage at the select gate 40, it allows the current in the T1's portionof the channel to pass between the source and drain. The selecttransistor provides a switch along the source-drain channel independentof the voltage at the control gate. One advantage is that it can be usedto turn off those cells that are still conducting at zero control gatevoltage due to their charge depletion (positive) at their floatinggates. The other advantage is that it allows source side injectionprogramming to be more easily implemented.

One simple embodiment of the split-channel memory cell is where theselect gate and the control gate are connected to the same word line asindicated schematically by a dotted line shown in FIG. 1B. This isaccomplished by having a charge storage element (floating gate)positioned over one portion of the channel and a control gate structure(which is part of a word line) positioned over the other channel portionas well as over the charge storage element. This effectively forms acell with two transistors in series, one (the memory transistor) with acombination of the amount of charge on the charge storage element andthe voltage on the word line controlling the amount of current that canflow through its portion of the channel, and the other (the selecttransistor) having the word line alone serving as its gate. Examples ofsuch cells, their uses in memory systems and methods of manufacturingthem are given in U.S. Pat. Nos. 5,070,032, 5,095,344, 5,315,541,5,343,063, and 5,661,053.

A more refined embodiment of the split-channel cell shown in FIG. 1B iswhen the select gate and the control gate are independent and notconnected by the dotted line between them. One implementation has thecontrol gates of one column in an array of cells connected to a control(or steering) line perpendicular to the word line. The effect is torelieve the word line from having to perform two functions at the sametime when reading or programming a selected cell. Those two functionsare (1) to serve as a gate of a select transistor, thus requiring aproper voltage to turn the select transistor on and off, and (2) todrive the voltage of the charge storage element to a desired levelthrough an electric field (capacitive) coupling between the word lineand the charge storage element. It is often difficult to perform both ofthese functions in an optimum manner with a single voltage. With theseparate control of the control gate and the select gate, the word lineneed only perform function (1), while the added control line performsfunction (2). This capability allows for design of higher performanceprogramming where the programming voltage is geared to the targeteddata. The use of independent control (or steering) gates in a flashEEPROM array is described, for example, in U.S. Pat. Nos. 5,313,421 and6,222,762.

FIG. 1C illustrates schematically another flash EEPROM cell having dualfloating gates and independent select and control gates. The memory cell10 is similar to that of FIG. 1B except it effectively has threetransistors in series. In this type of cell, two storage elements (i.e.,that of T1—left and T1—right) are included over its channel betweensource and drain diffusions with a select transistor T1 in between them.The memory transistors have floating gates 20 and 20′, and control gates30 and 30′, respectively. The select transistor T2 is controlled by aselect gate 40. At any one time, only one of the pair of memorytransistors is accessed for read or write. When the storage unit T1—leftis being accessed, both the T2 and T1—right are turned on to allow thecurrent in the T1—left's portion of the channel to pass between thesource and the drain. Similarly, when the storage unit T1—right is beingaccessed, T2 and T1—left are turned on. Erase is effected by having aportion of the select gate polysilicon in close proximity to thefloating gate and applying a substantial positive voltage (e.g. 20V) tothe select gate so that the electrons stored within the floating gatecan tunnel to the select gate polysilicon.

FIG. 1D illustrates schematically a string of memory cells organizedinto an NAND cell. An NAND cell 50 consists of a series of memorytransistors M1, M2, . . . Mn (n=4, 8, 16 or higher) daisy-chained bytheir sources and drains. A pair of select transistors S1, S2 controlsthe memory transistors chain's connection to the external via the NANDcell's source terminal 54 and drain terminal 56. In a memory array, whenthe source select transistor S1 is turned on, the source terminal iscoupled to a source line. Similarly, when the drain select transistor S2is turned on, the drain terminal of the NAND cell is coupled to a bitline of the memory array. Each memory transistor in the chain has acharge storage element to store a given amount of charge so as torepresent an intended memory state. A control gate of each memorytransistor provides control over read and write operations. A controlgate of each of the select transistors S1, S2 provides control access tothe NAND cell via its source terminal 54 and drain terminal 56respectively.

When an addressed memory transistor within an NAND cell is read andverified during programming, its control gate is supplied with anappropriate voltage. At the same time, the rest of the non-addressedmemory transistors in the NAND cell 50 are fully turned on byapplication of sufficient voltage on their control gates. In this way, aconductive path is effective created from the source of the individualmemory transistor to the source terminal 54 of the NAND cell andlikewise for the drain of the individual memory transistor to the drainterminal 56 of the cell. Memory devices with such NAND cell structuresare described in U.S. Pat. Nos. 5,570,315, 5,903,495, 6,046,935.

FIG. 1E illustrates schematically a non-volatile memory with adielectric layer for storing charge. Instead of the conductive floatinggate elements described earlier, a dielectric layer is used. Such memorydevices utilizing dielectric storage element have been described byEitan et al., “NROM: A Novel Localized Trapping, 2-Bit NonvolatileMemory Cell,” IEEE Electron Device Letters, vol. 21, no. 11, November2000, pp. 543-545. An ONO dielectric layer extends across the channelbetween source and drain diffusions. The charge for one data bit islocalized in the dielectric layer adjacent to the drain, and the chargefor the other data bit is localized in the dielectric layer adjacent tothe source. For example, U.S. Pat. Nos. 5,768,192 and 6,011,725 disclosea nonvolatile memory cell having a trapping dielectric sandwichedbetween two silicon dioxide layers. Multi-state data storage isimplemented by separately reading the binary states of the spatiallyseparated charge storage regions within the dielectric.

Memory Array

A memory device typically comprises of a two-dimensional array of memorycells arranged in rows and columns and addressable by word lines and bitlines. The array can be formed according to an NOR type or an NAND typearchitecture.

NOR Array

FIG. 2 illustrates an example of an NOR array of memory cells. Memorydevices with an NOR type architecture have been implemented with cellsof the type illustrated in FIGS. 1B or 1C. Each row of memory cells areconnected by their sources and drains in a daisy-chain manner. Thisdesign is sometimes referred to as a virtual ground design. Each memorycell 10 has a source 14, a drain 16, a control gate 30 and a select gate40. The cells in a row have their select gates connected to word line42. The cells in a column have their sources and drains respectivelyconnected to selected bit lines 34 and 36. In some embodiments where thememory cells have their control gate and select gate controlledindependently, a steering line 36 also connects the control gates of thecells in a column.

Many flash EEPROM devices are implemented with memory cells where eachis formed with its control gate and select gate connected together. Inthis case, there is no need for steering lines and a word line simplyconnects all the control gates and select gates of cells along each row.Examples of these designs are disclosed in U.S. Pat. Nos. 5,172,338 and5,418,752. In these designs, the word line essentially performed twofunctions: row selection and supplying control gate voltage to all cellsin the row for reading or programming.

NAND Array

FIG. 3 illustrates an example of an NAND array of memory cells, such asthat shown in FIG. 1D. Along each column of NAND cells, a bit line iscoupled to the drain terminal 56 of each NAND cell. Along each row ofNAND cells, a source line may connect all their source terminals 54.Also the control gates of the NAND cells along a row are connected to aseries of corresponding word lines. An entire row of NAND cells can beaddressed by turning on the pair of select transistors (see FIG. 1D)with appropriate voltages on their control gates via the connected wordlines. When a memory transistor within the chain of a NAND cell is beingread, the remaining memory transistors in the chain are turned on hardvia their associated Word lines so that the current flowing through thechain is essentially dependent upon the level of charge stored in thecell being read. An example of an NAND architecture array and itsoperation as part of a memory system is found in U.S. Pat. Nos.5,570,315, 5,774,397 and 6,046,935.

Block Erase

Programming of charge storage memory devices can only result in addingmore charge to its charge storage elements. Therefore, prior to aprogram operation, existing charge in a charge storage element must beremoved (or erased). Erase circuits (not shown) are provided to eraseone or more blocks of memory cells. A non-volatile memory such as EEPROMis referred to as a “Flash” EEPROM when an entire array of cells, orsignificant groups of cells of the array, is electrically erasedtogether (i.e., in a flash). Once erased, the group of cells can then bereprogrammed. The group of cells erasable together may consist one ormore addressable erase unit. The erase unit or block typically storesone or more pages of data, the page being the unit of programming andreading, although more than one page may be programmed or read in asingle operation. Each page typically stores one or more sectors ofdata, the size of the sector being defined by the host system. Anexample is a sector of 512 bytes of user data, following a standardestablished with magnetic disk drives, plus some number of bytes ofoverhead information about the user data and/or the block in with it isstored.

Read/Write Circuits

In the usual two-state EEPROM cell, at least one current breakpointlevel is established so as to partition the conduction window into tworegions. When a cell is read by applying predetermined, fixed voltages,its source/drain current is resolved into a memory state by comparingwith the breakpoint level (or reference current I_(REF)). If the currentread is higher than that of the breakpoint level, the cell is determinedto be in one logical state (e.g., a “zero” state). On the other hand, ifthe current is less than that of the breakpoint level, the cell isdetermined to be in the other logical state (e.g., a “cone” state).Thus, such a two-state cell stores one bit of digital information. Areference current source, which may be externally programmable, is oftenprovided as part of a memory system to generate the breakpoint levelcurrent.

In order to increase memory capacity, flash EEPROM devices are beingfabricated with higher and higher density as the state of thesemiconductor technology advances. Another method for increasing storagecapacity is to have each memory cell store more than two states.

For a multi-state or multi-level EEPROM memory cell, the conductionwindow is partitioned into more than two regions by more than onebreakpoint such that each cell is capable of storing more than one bitof data. The information that a given EEPROM array can store is thusincreased with the number of states that each cell can store. EEPROM orflash EEPROM with multi-state or multi-level memory cells have beendescribed in U.S. Pat. No. 5,172,338.

In practice, the memory state of a cell is usually read by sensing theconduction current across the source and drain electrodes of the cellwhen a reference voltage is applied to the control gate. Thus, for eachgiven charge on the floating gate of a cell, a corresponding conductioncurrent with respect to a fixed reference control gate voltage may bedetected. Similarly, the range of charge programmable onto the floatinggate defines a corresponding threshold voltage window or a correspondingconduction current window.

Alternatively, instead of detecting the conduction current among apartitioned current window, it is possible to set the threshold voltagefor a given memory state under test at the control gate and detect ifthe conduction current is lower or higher than a threshold current. Inone implementation the detection of the conduction current relative to athreshold current is accomplished by examining the rate the conductioncurrent is discharging through the capacitance of the bit line.

FIG. 4 illustrates the relation between the source-drain current ID andthe control gate voltage V_(CG) for four different charges Q1-Q4 thatthe floating gate may be selectively storing at any one time. The foursolid I_(D) versus V_(CG) curves represent four possible charge levelsthat can be programmed on a floating gate of a memory cell, respectivelycorresponding to four possible memory states. As an example, thethreshold voltage window of a population of cells may range from 0.5V to3.5V. Six memory states may be demarcated by partitioning the thresholdwindow into five regions in interval of 0.5V each. For example, if areference current, I_(REF) of 2 μA is used as shown, then the cellprogrammed with Q1 may be considered to be in a memory state “1” sinceits curve intersects with I_(REF) in the region of the threshold windowdemarcated by V_(CG)=0.5V and 1.0V. Similarly, Q4 is in a memory state“5”.

As can be seen from the description above, the more states a memory cellis made to store, the more finely divided is its threshold window. Thiswill require higher precision in programming and reading operations inorder to be able to achieve the required resolution.

U.S. Pat. No. 4,357,685 discloses a method of programming a 2-stateEPROM in which when a cell is programmed to a given state, it is subjectto successive programming voltage pulses, each time adding incrementalcharge to the floating gate. In between pulses, the cell is read back orverified to determine its source-drain current relative to thebreakpoint level. Programming stops when the current state has beenverified to reach the desired state. The programming pulse train usedmay have increasing period or amplitude.

Prior art programming circuits simply apply programming pulses to stepthrough the threshold window from the erased or ground state until thetarget state is reached. Practically, to allow for adequate resolution,each partitioned or demarcated region would require at least about fiveprogramming steps to transverse. The performance is acceptable for2-state memory cells. However, for multi-state cells, the number ofsteps required increases with the number of partitions and therefore,the programming precision or resolution must be increased. For example,a 16-state cell may require on average at least 40 programming pulses toprogram to a target state.

FIG. 5 illustrates schematically a memory device with a typicalarrangement of a memory array 100 accessible by read/write circuits 170via row decoder 130 and column decoder 160. As described in connectionwith FIGS. 2 and 3, a memory transistor of a memory cell in the memoryarray 100 is addressable via a set of selected word line(s) and bitline(s). The row decoder 130 selects one or more word lines and thecolumn decoder 160 selects one or more bit lines in order to applyappropriate voltages to the respective gates of the addressed memorytransistor. Read/write circuits 170 are provided to read or writeprogram) the memory states of addressed memory transistors. Theread/write circuits 170 comprise a number of read/write modulesconnectable via bit lines to memory elements in the array.

FIG. 6A is a schematic block diagram of an individual read/write module190. Essentially, during read or verify, a sense amplifier determinesthe current flowing through the drain of an addressed memory transistorconnected via a selected bit line. The current depends on the chargestored in the memory transistor and its control gate voltage. Forexample, in a multi-state EEPROM cell, its floating gate can be chargedto one of several different levels. For a 4-level cell, it may he usedto store two bits of data. The level detected by the sense amplifier isconverted by a level-to-bits conversion logic to a set of data bits tobe stored in a data latch.

Factors Affecting Read/Write Performance and Accuracy

In order to improve read and program performance, multiple chargestorage elements or memory transistors in an array are read orprogrammed in parallel. Thus, a logical “page” of memory elements areread or programmed together. In existing memory architectures, a rowtypically contains several interleaved pages. All memory elements of apage will be read or programmed together. The column decoder willselectively connect each one of the interleaved pages to a correspondingnumber of read/write modules. For example, in one implementation, thememory array is designed to have a page size of 532 bytes (512 bytesplus 20 bytes of overheads.) If each column contains a drain bit lineand there are two interleaved pages per row, this amounts to 8512columns with each page being associated with 4256 columns. There will be4256 sense modules connectable to read or write in parallel either allthe even bit lines or the odd bit lines. In this way, a page of 4256bits (i.e., 532 bytes) of data in parallel are read from or programmedinto the page of memory elements. The read/write modules forming theread/write circuits 176 can be arranged into various architectures.

Referring to FIG. 5, the read/write circuits 170 is organized into banksof read/write stacks 180. Each read/write stack 180 is a stack ofread/write modules 190. In a memory array, the column spacing isdetermined by the size of the one or two transistors that occupy it.However, as can be seen from FIG. 6A, the circuitry of a read/writemodule will likely be implemented with many more transistors and circuitelements and therefore will occupy a space over many columns. In orderto service more than one column among the occupied columns, multiplemodules are stacked up on top of each other.

FIG. 6B shows the read/write stack of FIG. 5 implemented conventionallyby a stack of read/write modules 190. For example, a read/write modulemay extend over sixteen columns, then a read/write stack 180 with astack of eight read/write modules can be used to service eight columnsin parallel. The read/write stack can be coupled via a column decoder toeither the eight odd (1, 3, 5, 7, 9, 11, 13, 15) columns or the eighteven (2, 4, 6, 8, 10, 12, 14, 16) columns among the bank.

As mentioned before, conventional memory devices improve read/writeoperations by operating in a massively parallel manner on all even orall odd bit lines at a time. This architecture of a row consisting oftwo interleaved pages will help to alleviate the problem of fitting theblock of read/write circuits. It is also dictated by consideration ofcontrolling bit-line to bit-line capacitive coupling. A block decoder isused to multiplex the set of read/write modules to either the even pageor the odd page. In this way, whenever one set bit lines are being reador programmed, the interleaving set can be grounded to minimizeimmediate neighbor coupling.

However, the interleaving page architecture is disadvantageous in atleast three respects. First, it requires additional multiplexingcircuitry. Secondly, it is slow in performance. To finish read orprogram of memory cells connected by a word line or in a row, two reador two program operations are required. Thirdly, it is also not optimumin addressing other disturb effects such as field coupling betweenneighboring charge storage elements at the floating gate level when thetwo neighbors are programmed at different times, such as separately inodd and even pages.

The problem of neighboring field coupling becomes more pronounced withever closer spacing between memory transistors. In a memory transistor,a charge storage element is sandwiched between a channel region and acontrol gate. The current that flows in the channel region is a functionof the resultant electric field contributed by the field at the controlgate and the charge storage element. With ever increasing density,memory transistors are formed closer and closer together. The field fromneighboring charge elements then becomes significant contributor to theresultant field of an affected cell. The neighboring field depends onthe charge programmed into the charge storage elements of the neighbors.This perturbing field is dynamic in nature as it changes with theprogrammed states of the neighbors. Thus, an affected cell may readdifferently at different time depending on the changing states of theneighbors.

The conventional architecture of interleaving page exacerbates the errorcaused by neighboring floating gate coupling. Since the even page andthe odd page are programmed and read independently of each other, a pagemay be programmed under one set of condition but read back under anentirely different set of condition, depending on what has happened tothe intervening page in the meantime. The read errors will become moresevere with increasing density, requiring a more accurate read operationand coarser partitioning of the threshold window for multi-stateimplementation. Performance will suffer and the potential capacity in amulti-state implementation is limited.

United States Patent Publication No. US-2004-0060031-A1 discloses a highperformance yet compact non-volatile memory device having a large blockof read/write circuits to read and write a corresponding block of memorycells in parallel. In particular, the memory device has an architecturethat reduces redundancy in the block of read/write circuits to aminimum. Significant saving in space as well as power is accomplished byredistributing the block of read/write modules into a block read/writemodule core portions that operate in parallel while interacting with asubstantially smaller sets of common portions in a time-multiplexingmanner. In particular, data processing among read/write circuits betweena plurality of sense amplifiers and data latches is performed by ashared processor.

Therefore there is a general need for high performance and high capacitynon-volatile memory. In particular, there is a need for a compactnon-volatile memory with enhanced read and program performance having animproved processor that is compact and efficient, yet highly versatilefor processing data among the read/writing circuits.

SUMMARY OF INVENTION

A non-volatile memory circuit including an array of non-volatile memorycells formed along columns of multiple bits, the columns including aplurality of regular columns and one or more redundancy columns, isdescribed. The memory circuit also includes a plurality of latches, eachcorresponding to one of the regular columns and having a bit whose valueindicates if the corresponding column is defective. The memory circuitstoring a column redundancy data table whose contents indicate for eachredundancy column whether the redundancy column is being used and, forredundancy columns that are being used, a defective regular column towhich it corresponds and the bits therein which are defective. Thememory circuit stores data corresponding to the defective bits ofdefective regular columns in the redundancy column portion.

According to an additional set of aspects, a method of operating anon-volatile memory circuit is presented, where the memory circuitincludes an array of non-volatile memory cells formed along columns ofmultiple bits and having a latch associated with each of the columnswhose value indicates if the corresponding column has a defect. Themethod includes: performing a write operation to concurrently program aplurality of memory cells on a corresponding plurality of columns,including one or more columns having an associated latch whose valueindicates the corresponding column has a defect; determining the numberof the plurality of concurrently programmed memory cells that were notsuccessfully programmed in the write operation, wherein the columnswhose latch values indicate the column has a defect are not counted inthe determining; and determining whether the number of cells that werenot successfully been programmed during the write operation isacceptable.

According to another set of aspects, methods of operating a non-volatilememory circuit having an array of non-volatile memory cells formed alongcolumns of multiple bits, the columns including a plurality of regularcolumns and one or more redundancy columns are presented. The methodincludes performing a plurality of column test operations to determinewhich columns are defective and the individual bits therein which aredefective, each of the column tests including: writing and reading backan externally supplied data pattern to the columns; and comparing theexternally supplied data pattern as read back with an expected datapattern, wherein said column test operation are performed by circuitryon the memory circuit and each of the column tests uses a different datapattern. The method also includes recording addresses of any of theregular columns determined defective and the individual bits thereinwhich are determined defective in a column redundancy data table storedon the memory circuit; and, for any of the regular columns determineddefective, setting a latch associated therewith to a value indicatingthat the associated column is defective.

In other aspects, a method of operating a non-volatile memory circuithaving an array of non-volatile memory cells formed along columns ofmultiple bits, the columns including a plurality of regular columns andone or more redundancy columns is described. The method includes:storing on the memory circuit a column redundancy data table whosecontents indicate for each redundancy column whether the redundancycolumn is being used and, for redundancy columns that are being used, adefective regular column to which it corresponds and the bits thereinwhich are defective; receiving a set of data to program into the memoryarray; determining the elements of the set of data assigned to beprogrammed to defective bits of defective regular columns based upon thecolumn redundancy circuit data table; storing the elements of the set ofdata determined to be assigned to be programmed to defective bits ofdefective columns in peripheral latch circuits on the memory circuit;storing the set of data into programming latches for the memory array;performing a programming operation into the regular columns of thememory array from the programming latches; and programming the elementsof the data set stored in the peripheral latches into the redundancycolumns.

Various aspects, advantages, features and embodiments of the presentinvention are included in the following description of exemplaryexamples thereof which description should be taken in conjunction withthe accompanying drawings. All patents, patent applications, articles,other publications, documents and things referenced herein are herebyincorporated herein by this reference in their entirety for allpurposes. To the extent of any inconsistency or conflict in thedefinition or use of terms between any of the incorporated publications,documents or things and the present application, those of the presentapplication shall prevail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E illustrate schematically different examples of non-volatilememory cells.

FIG. 2 illustrates an example of an NOR array of memory cells.

FIG. 3 illustrates an example of an NAND array of memory cells, such asthat shown in FIG. 1D.

FIG. 4 illustrates the relation between the source-drain current and thecontrol gate voltage for four different charges Q1-Q4 that the floatinggate may be storing at any one time.

FIG. 5 illustrates schematically a typical arrangement of a memory arrayaccessible by read/write circuits via row and column decoders.

FIG. 6A is a schematic block diagram of an individual read/write module.

FIG. 6B shows the read/write stack of FIG. 5 implemented conventionallyby a stack of read/write modules.

FIG. 7A illustrates schematically a compact memory device having a bankof partitioned read/write stacks, in which the improved processor of thepresent invention is implemented.

FIG. 7B illustrates a preferred arrangement of the compact memory deviceshown in FIG. 7A.

FIG. 8 illustrates schematically a general arrangement of the basiccomponents in a read/write stack shown in FIG. 7A.

FIG. 9 illustrates one preferred arrangement of the read/write stacksamong the read/write circuits shown in FIGS. 7A and 7B.

FIG. 10 illustrates an improved embodiment of the common processor shownin FIG. 9.

FIG. 11A illustrates a preferred embodiment of the input logic of thecommon processor shown in FIG. 10.

FIG. 11B illustrates the truth table of the input logic of FIG. 11A.

FIG. 12A illustrates a preferred embodiment of the output logic of thecommon processor shown in FIG. 10.

FIG. 12B illustrates the truth table of the output logic of FIG. 12A.

FIG. 13 illustrates an example of a format for column redundancy datawithout bit information.

FIG. 14A illustrates an example of a format for column redundancy dataincluding bit information.

FIG. 14B illustrates an alternate embodiment of a format for columnredundancy data including bit information.

FIGS. 15 and 16 respectively give a schematic representation of bitsubstitution in the write and read process.

FIG. 17 is an exemplary flow for a built in self-test algorithm.

FIGS. 18-20 show some examples of circuitry that can be used toimplement some of the elements of the flow of FIG. 17.

FIG. 21 is a schematic representation of the on-chip management for badbits.

FIGS. 22A and 22 b are examples of data latches that could be used fordata compactification.

FIGS. 23 and 25 respectively illustrate a set of bad bits before andafter compacting.

FIGS. 24 and 26 respectively illustrate an arrangement of latches forpacking and unpacking the data corresponding to the bad bits.

FIGS. 27 and 28 show some exemplary circuitry to implement elements forFIG. 26.

FIG. 29 show how bad bits can be extracted from the column redundancyinformation.

FIG. 30 illustrates an on-chip data folding process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 7A illustrates schematically a compact memory device having a bankof partitioned read/write stacks, in which the improved processor of thepresent invention is implemented. The memory device includes atwo-dimensional array of memory cells 300, control circuitry 310, andread/write circuits 370. The memory array 300 is addressable by wordlines via a row decoder 330 and by bit lines via a column decoder 360.The read/write circuits 370 is implemented as a bank of partitionedread/write stacks 400 and allows a block (also referred to as a “page”)of memory cells to be read or programmed in parallel. In a preferredembodiment, a page is constituted from a contiguous row of memory cells.In another embodiment, where a row of memory cells are partitioned intomultiple blocks or pages, a block multiplexer 350 is provided tomultiplex the read/write circuits 370 to the individual blocks.

The control circuitry 310 cooperates with the read/write circuits 370 toperform memory operations on the memory array 300. The control circuitry310 includes a state machine 312, an on-chip address decoder 314 and apower control module 316. The state machine 312 provides chip levelcontrol of memory operations. The on-chip address decoder 314 providesan address interface between that used by the host or a memorycontroller to the hardware address used by the decoders 330 and 370. Thepower control module 316 controls the power and voltages supplied to theword lines and bit lines during memory operations.

FIG. 7B illustrates a preferred arrangement of the compact memory deviceshown in FIG. 7A. Access to the memory array 300 by the variousperipheral circuits is implemented in a symmetric fashion, on oppositesides of the array so that access lines and circuitry on each side arereduced in half. Thus, the row decoder is split into row decoders 330Aand 330B and the column decoder into column decoders 360A and 360B. Inthe embodiment where a row of memory cells are partitioned into multipleblocks, the block multiplexer 350 is split into block multiplexers 350Aand 350B. Similarly, the read/write circuits are split into read/writecircuits 370A connecting to bit lines from the bottom and read/writecircuits 370B connecting to bit lines from the top of the array 300. Inthis way, the density of the read/write modules, and therefore that ofthe partitioned read/write stacks 400, is essentially reduced by onehalf.

FIG. 8 illustrates schematically a general arrangement of the basiccomponents in a read/write stack shown in FIG. 7A. According to ageneral architecture of the invention, the read/write stack 400comprises a stack of sense amplifiers 212 for sensing k bit lines, anI/O module 440 for input or output of data via an I/O bus 231, a stackof data latches 430 for storing input or output data, a common processor500 to process and store data among the read/write stack 400, and astack bus 421 for communication among the stack components. A stack buscontroller among the read/write circuits 370 provides control and timingsignals via lines 411 for controlling the various components among theread/write stacks.

FIG. 9 illustrates one preferred arrangement of the read/write stacksamong the read/write circuits shown in FIGS. 7A and 7B. Each read/writestack 400 operates on a group of k bit lines in parallel. If a page hasp=r*k bit lines, there will be r read/write stacks, 400-1, . . . ,400-r.

The entire bank of partitioned read/write stacks 400 operating inparallel allows a block (or page) of p cells along a row to be read orprogrammed in parallel. Thus, there will be p read/write modules for theentire row of cells. As each stack is serving k memory cells, the totalnumber of read/write stacks in the bank is therefore given by r=p/k. Forexample, if r is the number of stacks in the bank, then p=r*k. Oneexample memory array may have p=512 bytes (512×8 bits), k=8, andtherefore r=512. In the preferred embodiment, the block is a run of theentire row of cells. In another embodiment, the block is a subset ofcells in the row. For example, the subset of cells could be one half ofthe entire row or one quarter of the entire row. The subset of cellscould be a run of contiguous cells or one every other cell, or one everypredetermined number of cells.

Each read/write stack, such as 400-1, essentially contains a stack ofsense amplifiers 212-1 to 212-k servicing a segment of k memory cells inparallel. A preferred sense amplifier is disclosed in United StatesPatent Publication No. 2004-0109357-A1, the entire disclosure of whichis hereby incorporated herein by reference.

The stack bus controller 410 provides control and timing signals to theread/write circuit 370 via lines 411. The stack bus controller is itselfdependent on the memory controller 310 via lines 311. Communicationamong each read/write stack 400 is effected by an interconnecting stackbus 431 and controlled by the stack bus controller 410. Control lines411 provide control and clock signals from the stack bus controller 410to the components of the read/write stacks 400-1.

In the preferred arrangement, the stack bus is partitioned into a SABus422 for communication between the common processor 500 and the stack ofsense amplifiers 212, and a DBus 423 for communication between theprocessor and the stack of data latches 430.

The stack of data latches 430 comprises of data latches 430-1 to 430-k,one for each memory cell associated with the stack The I/O module 440enables the data latches to exchange data with the external via an I/Obus 231.

The common processor also includes an output 507 for output of a statussignal indicating a status of the memory operation, such as an errorcondition. The status signal is used to drive the gate of ann-transistor 550 that is tied to a FLAG BUS 509 in a Wired-Orconfiguration. The FLAG BUS is preferably precharged by the controller310 and will be pulled down when a status signal is asserted by any ofthe read/write stacks. (The isolation latch IL 529 is discussed in thefollowing section on bad column management.)

FIG. 10 illustrates an improved embodiment of the common processor shownin FIG. 9. The common processor 500 comprises a processor bus, PBUS 505for communication with external circuits, an input logic 510, aprocessor latch PLatch 520 and an output logic 530.

The input logic 510 receives data from the PBUS and outputs to a BS1node as a transformed data in one of logical states “1”, “0”, or “Z”(float) depending on the control signals from the stack bus controller410 via signal lines 411. A Set/Reset latch, PLatch 520 then latchesBSI, resulting in a pair of complementary output signals as MTCH andMTCH*.

The output logic 530 receives the MTCH and MTCH* signals and outputs onthe PBUS 505 a transformed data in one of logical states “1”, “0”, or“Z” (float) depending on the control signals from the stack buscontroller 410 via signal lines 411.

At any one time the common processor 500 processes the data related to agiven memory cell. For example, FIG. 10 illustrates the case for thememory cell coupled to bit line 1. The corresponding sense amplifier212-1 comprises a node where the sense amplifier data appears. In thepreferred embodiment, the node assumes the form of a SA Latch, 214-1that stores data. Similarly, the corresponding set of data latches 430-1stores input or output data associated with the memory cell coupled tobit line 1. In the preferred embodiment, the set of data latches 430-1comprises sufficient data latches, 434-1, . . . , 434-n for storingn-bits of data.

The PBUS 505 of the common processor 500 has access to the SA latch214-1 via the SBUS 422 when a transfer gate 501 is enabled by a pair ofcomplementary signals SAP and SAN. Similarly, the PBUS 505 has access tothe set of data latches 430-1 via the DBUS 423 when a transfer gate 502is enabled by a pair of complementary signals DTP and DTN. The signalsSAP, SAN, DTP and DTN are illustrated explicitly as part of the controlsignals from the stack bus controller 410.

FIG. 11A illustrates a preferred embodiment of the input logic of thecommon processor shown in FIG. 10. The input logic 520 receives the dataon the PBUS 505 and depending on the control signals, either has theoutput BS1 being the same, or inverted, or floated. The output BS1 nodeis essentially affected by either the output of a transfer gate 522 or apull-up circuit comprising p-transistors 524 and 525 in series to Vdd,or a pull-down circuit comprising n-transistors 526 and 527 in series toground. The pull-up circuit has the gates to the p-transistor 524 and525 respectively controlled by the signals PBUS and ONE. The pull-downcircuit has the gates to the n-transistors 526 and 527 respectivelycontrolled by the signals ONEB<1> and PBUS.

FIG. 11B illustrates the trith table of the input logic of FIG. 11A. Thelogic is controlled by PBUS and the control signals ONE, ONEB<0>,ONEB<1> which are part of the control signals from the stack buscontroller 410. Essentially, three transfer modes, PASSTHROUGH,INVERTED, and FLOATED, are supported.

In the case of the PASSTHROUGH mode where BS1 is the same as the inputdata, the signals ONE is at a logical “1”, ONEB<0> at “0” and ONEB<1> at“0”. This will disable the pull-up or pull-down but enable the transfergate 522 to pass the data on the PBUS 505 to the output 523. In the caseof the INVERTED mode where BS1 is the invert of the input data, thesignals ONE is at “0”, ONEB<0> at “1” and ONE<1> at “1”. This willdisable the transfer gate 522. Also, when PBUS is at “0”, the pull-downcircuit will be disabled while the pull-up circuit is enabled, resultingin BS1 being at “1”. Similarly, when PBUS is at “1”, the pull-up circuitis disabled while the pull-down circuit is enabled, resulting in BS1being at “0”. Finally, in the case of the FLOATED mode, the output BS1can be floated by having the signals ONE at “1”, ONEB<0> at “1” andONEB<1> at “0”. The FLOATED mode is listed for completeness although inpractice, it is not used.

FIG. 12A illustrates a preferred embodiment of the output logic of thecommon processor shown in FIG. 10. The signal at the BS1 node from theinput logic 520 is latched in the processor latch, PLatch 520. Theoutput logic 530 receives the data MTCH and MTCH* from the output ofPLatch 520 and depending on the control signals, outputs on the PBUS aseither in a PASSTHROUGH, INVERTED OR FLOATED mode. In other words, thefour branches act as drivers for the PBUS 505, actively pulling iteither to a HIGH, LOW or FLOATED state. This is accomplished by fourbranch circuits, namely two pull-up and two pull-down circuits for thePBUS 505. A first pull-up circuit comprises p-transistors 531 and 532 inseries to Vdd, and is able to pull up the PBUS when MTCH is at “0”. Asecond pull-up circuit comprises p-transistors 533 and 534 in series toground and is able to pull up the PBUS when MTCH is at “1”. Similarly, afirst pull-down circuit comprises n-transistors 535 and 536 in series toVdd, and is able to pull down the PBUS when MTCH is at “0”. A secondpull-up circuit comprises n-transistors 537 and 538 in series to groundand is able to pull up the PBUS when MTCH is at “1”.

One feature of the invention is to constitute the pull-up circuits withPMOS transistors and the pull-down circuits with NMOS transistors. Sincethe pull by the NMOS is much stronger than that of the PMOS, thepull-down will always overcome the pull-up in any contentions. In otherwords, the node or bus can always default to a pull-up or “1” state, andif desired, can always be flipped to a “0” state by a pull-down.

FIG. 12B illustrates the truth table of the output logic of FIG. 12A.The logic is controlled by MTCH, MTCH* latched from the input logic andthe control signals PDIR, PINV, NDIR, NINV, which are part of thecontrol signals from the stack bus controller 410. Four operation modes,PASSTHROUGH, INVERTED, FLOATED, and PRECHARGE are supported.

In the FLOATED mode, all four branches are disabled. This isaccomplished by having the signals PINV=1, NINV=0, PDIR=1, NDIR=0, whichare also the default values. In the PASSTHROUGH mode, when MTCH=0, itwill require PBUS=0. This is accomplished by only enabling the pull-downbranch with n-transistors 535 and 536, with all control signals at theirdefault values except for NDIR=1. When MTCH=1, it will require PBUS=1.This is accomplished by only enabling the pull-up branch withp-transistors 533 and 534, with all control signals at their defaultvalues except for PINV=0. In the INVERTED mode, when MTCH=0, it willrequire PBUS=1. This is accomplished by only enabling the pull-up branchwith p-transistors 531 and 532, with all control signals at theirdefault values except for PDIR=0. When MTCH=1, it will require PBUS=0.This is accomplished by only enabling the pull-down branch withn-transistors 537 and 538, with all control signals at their defaultvalues except for NINV=1. In the PRECHARGE mode, the control signalssettings of PDIR=0 and PINV=0 will either enable the pull-up branch withp-transistors 531 and 531 when MTCH=1 or the pull-up branch withp-transistors 533 and 534 when MTCH=0.

Common processor operations are developed more fully in U.S. patentapplication Ser. No. 11/026,536, Dec. 29, 2004, which is herebyincorporated in its entirety by this reference.

Bad Column Management with Bit Information

A memory will often have defective portions, either from themanufacturing process or that arise during the operation of the device.A number of techniques exist for managing these defects including errorcorrection coding or remapping portions of the memory, such as describedin U.S. Pat. Nos. 7,405,985, 5,602,987, 5,315,541, 5,200,959, and5,428,621. For instance, a device is generally thoroughly tested beforebeing shipped. The testing may find a defective portion of the memorythat needs to be eliminated. Before shipping the device, the informationon these defects is stored on the device, for example in a ROM area ofthe memory array or in a separate ROM, and at power up it is read by acontroller and then used so that the controller can substitute a goodportion of the memory for the bad. When reading or writing, thecontroller will then need to refer to a pointer structure in thecontroller's memory for this remapping.

In previous arrangements for managing bad columns, such as in U.S. Pat.No. 7,405,985, when there is an error in a column, the whole column istypically mapped out, with the corresponding whole byte or word will bemarked to be bad. According to the aspects presented in this section,the system can detect when only I bit in the byte is bad and bytes withsingle bit failures can be utilized as long as the single bit is savedelsewhere in the memory. Through the analysis of the any defectivecolumns, it can be determined whether they are in the category where thewhole will be treated as bad or whether it only has only single bitfailures so that the other bits in the bad columns can be used as good.In an exemplary application, during the die sort, those single bitfailures and their column address as well as bit address can be detectedand saved in a non-volatile ROM block. When the controller manages thesebad columns by this information, the bit information can be used toextract the corresponding bits saved in a column redundancy area. Thecan consequently enhance the yield so that more defects can be repairedby the column redundancy, since columns with only single bit errors canstill be used, rather than mapped out.

More specifically, each column of the memory has an associated isolationlatch or register whose value indicates whether the column is defective,but in addition to this information, for columns marked as defective,additional information is used to indicate whether the column as a wholeis to be treated as defective, or whether just individual bits of thecolumn are defective. The defective elements can then be re-mapped to aredundant element at either the appropriate bit or column level based onthe data. When a column is bad, but only on the bit level, the good bitscan still be used for data, although this may be done at a penalty ofunder programming for some bits, as is described further below. In anexemplary embodiment, the bad column and bad bit information isdetermined as part of a self contained Built In Self Test (BIST) flowconstructed to collect the bit information through a set of columntests. Based on this information, the bad bits can be extracted andre-grouped into bytes by the controller or on the memory, depending onthe embodiment, to more efficiently use the column redundancy area.These techniques and structures can be applied to the various memoryarchitectures described above, including NOR architectures, NANDarchitectures, and even the sort of 3D memory structures described inU.S. patent application Ser. No. 12/414,935. When reference to aspecific memory architecture is useful, NAND flash memory will serve asthe exemplary embodiment.

Returning briefly to the case of where bad columns are managed withoutbit information, non-volatile memories usually have redundancy to repairon-chip failures. Column redundancy is used to repair the bad columns,where the repair unit is normally one byte as a unit, or sometimes aword as a unit. Under this arrangement, even for a 1 bit fail in the 1byte, the whole byte will be marked to be a bad column and the data willbe moved to the redundancy area. This is a convenient way to isolate thebad column as a group of bad bitlines, but the penalty is that theredundancy repair unit could be exhausted fairly rapidly. The bad columnaddress is normally saved in the ROM block of the non-volatile memory.In the exemplary embodiments below, there are 13 column addresses,A<13:1>. The format for column redundancy data can then use 2 bytes toremember one column address. There are 2 flag bits to indicate that itis a unused column redundancy, or a used column redundancy, or a Badcolumn redundancy, as shown in the table of FIG. 13. The reason toisolate at the one byte or one word level is that isolation latch takessome area, it will typically not be practical to have an isolation latchfor every bad bitline. In the exemplary arrangement used here, thepurpose of the isolation latch is to ignore the programming/erase resultof that byte or word. In an NAND-type architecture, operations are donein parallel where good and bad bits are done for each of the read,program, or erase simultaneously. In one particular embodiment, theisolation latch can be included as part of the common processor 500(FIG. 9) circuitry, where it is illustrated schematically as IL 529 inFIG. 9 and on the standard implementations of a latch circuit. As partof the common processor for the associated k bitlines, it can functionas described in the following. (As noted, the one latch in thisimplementation serves for the word or byte (k=8 or 16), rather anembodiment with a latch for each bit line, in which case there would besuch a latch associated with each bit line/sense amp 212 in FIG. 9.)This isolation latch is used in the case that the data latchesassociated with the sense amps are subjected to defects, since they aredrawn according to a tighter layout design rule. In the case that thedata latches could be guaranteed to be 100% perfect, the isolation latchis not necessary. In the latter case, the data in the bad bit will befilled with a data bit—a don't care data pattern; but the generalprinciple described here still applies: i. e. the bad bitline caused bythe memory array failure can be extracted from the bad bytes andre-grouped into a new byte with other bad bits and write to a new goodlocation in the memory.

FIG. 13 illustrates an example of a format for column redundancy datawithout bit information. The first two columns show the values of thetwo flag bits for an unused redundant column, a redundant column beingused, and a bad redundancy column. (The flag value of (1,0) is used hereand so an illegal case, but could be reserved for other cases.) For theembodiment shown here, the unit is taken as the word and the addressAA[1] distinguishes between the two bytes of the word, here referred toas the high and low byte. How each of the two of the format are used isthen shown to the right. In each of these cases, the two mostsignificant bits of the high byte hold the flag bits. For both an unusedcolumn and a bad column, there is not address to hold, so the remainingvalues are set to 1. If the redundant column is being used, the columnit is replacing can have its addressed stored as shown in the example.(As the example has 13 column address, two bytes are sufficient to holda column address and the two flag bits, where the number can be changedaccording to the number of column addresses the system uses.) When aredundant column is bad, it is also isolated and also marked to be badwith the flag. When a (non-redundant) column is bad, this will beindicated by the value of specific memory locations in the ROMpages/blocks on the non-volatile memory and/or an associated isolationlatch. The bad column information can be retrieved either at the poweron sequence or before each pages are operated on.

Bad columns can classified as one of two types: those such as an relatedto bitline short or open circuit, where there can be multiple bad bitfailure, and the whole column is taken as defective; and those such asdefects in the data latches or sense amps, which are typicallyindividual bit failures. To keep the physical array structure simple andsave on die size, the latch or register that indicates a column is bad(the isolation latch) uses one 1-bit latch per byte. (For architecturesthat have a top and bottom latch that would be isolated together, thenone defect will isolate 2 bytes (1 top, 1 bottom).) If the minimumrepair unit is taken as a byte or a word, this could cause inefficiencyin the management of bad columns, since, typically, most of the bits inthe bad columns are good bits which can be used.

It should be noted that when the isolation latch is set under thisarrangement, this does not mean the column is no longer accessible, justthat it is marked as “don't care” with respect to program or erasecompletion. Under this arrangement, columns that are defective on thebit level will have their isolation set and not counted among the goodcolumns; however, even though the bad columns are “isolated”, the cellswill get programmed (and erased ) and verified. At the end of a programoperation, however, at the isolation latch is set, any of their bitsthat have failed to program (slow bits) will not get counted as part thetotal failure count. Therefore, these bad columns do not participate inthe pseudo-pass criteria for programming (or erase) and there mayconsequently be some cells that are under-programmed (or under-erased)but un-detected. As these are slow cells in the normal good columns, thenumber of program (erase) pulses will be applied on the wordline to makesure that the data will be programmed (or erased) successfully.Additionally, as stronger FCC capability is available to thenon-volatile memory system, it allows for the system to take care ofmost of the slow bits.

For example, the system may have an allowance for 40 bits fail duringprogramming. Taking a programming operation as having, say, 9000 bytes,the ratio of failed bits is then 40/(9000*8). If 24 columns have beenreplaced with redundancy columns, where each byte has 1 bit bad bitline,and with 7 bits per byte programming without detection, then the numberof failed programmed bits will be {24*2*7*40/(9000*8)}=4 bits failure.The rest of the bits (24×7), besides the bad bitlines, in the bad columnwill be programmed correctly and these 4 bits can be managed by theerror correction code.

FIG. 14A shows a Column Redundancy Data (CRD) table format that includesbad bit information. As shown there, an extra pair of bytes will beadded to each bad column information shown as the lower pair of low,high rows. These will indicate which bits are bad, where the bad bit isindicated by “0”. The good bits will be indicated with “1”. For both theunused columns and the bad redundancy columns, this information is notrelevant and all the entries are set to “1”. For bad column where thewhole column is taken as bad (whole bit failure), all entries are set at“0” and this corresponds to the situation in FIG. 13. In the case ofsingle bit failure, the additional entries indicate which bits of thecolumn are bad, and need to be mapped out, and which bits are good. Inthis example, two bits (bit 6 of both bytes) are bad as indicated in bythe “0”, with the good bits having a “1”. (It just happens that bothbytes have bit 6 bad in this example and the bad bits need not line upin this way.)

In another embodiment, the mode of failures can be recorded in the badcolumn information. FIG. 14B shows an example where only one flagindicates a used redundancy column or a bad redundancy column. Mode0 andmode 1 are the two bits indicating the failure modes: 01—bitline open;10—bitline short; 11—data latch failure; 00—others cases. If two kindsof failure exist on the same byte (low probability case), only thelatest failure mode is recorded. The increase of the 2 bytes for eachbad column will not increase the die size, since the CRD data will besaved in one ROM block in the memory. ROM space usually is large enoughto save all the require information for bad column. The failure modeinformation may be used by the controller for various applications, forexample to digitally correct floating gate to floating gate capacitivecoupling effects that can occur in EEPROM based memories.

According to one aspect presented here, during die sort or the built inself-test (BIST) test flow discussed in the following, the bad columnscan be tested bit by bit in multiple column tests and failed bitinformation will be accumulated into a CRD table such as FIG. 14A or14B.

Thus, in the arrangement presented here, the number of failed bits canbe recorded in the one of these formats, which allows the columnredundancy data to record multiple bit failures for a column. The badcolumn can be managed by the memory circuits as well as controller. Forthe simplicity of presentation, the description here is mainly given forthe case when the controller manages the bad columns. Similar functioncan also be achieved by the circuits inside the non-volatile memory.During the program process, the controller will load the user-programdata intro the data latches inside the memory. The locationcorresponding to the bad bits can be left with user data or filled with“1”, but the copy of the data will also be saved in a good bit locationin the redundancy column area. As isolated bad columns with bit errorswill have some good data they will going through the program (or erase)process, and so the bad bit can just have their data latched for them aswell as in the remapped location. Regardless of the data in the badbitline, the operations can be done collectively on all cells withoutincreasing the power consumption in NAND flash architecture. In someother architecture, such as, NOR flash or 3D Read/Writable architecture,the bad bitlines are filled with data of non-operation to avoid extrapower loss.

The replacement of bad bits with good bits from the redundancy columnscan be illustrated schematically using FIGS. 15-16, which arerespectively a program situation, where the data is loaded to the normallocations and the bad column data is moved to the redundant column area,and a read situation, where the sensed data in redundant area is movedto the right location in the user bytes. As shown there, several of thecells (at addresses A2, A6, A8, A13, A15, A28) are defective and thereintended content is written into the redundancy section at left, wherethe same addresses are shown shaded. During the read process, the wholewordline data will be sensed to the data latches. The data may betransferred out to the controller. The controller side will fetch thegood bits from the redundant area and move them to the correct locationaccording to the bad column map table shown in FIG. 14A or 14B. Thisprocess is illustrated by FIG. 16, which is a sort of inverse of FIG.15, where the good bits in the redundancy section are read out andsubstituted for the defective cells they are standing in for. In FIGS.15 and 16, the Xs to the left, regular column area, indicated thedefects mapped into the redundancy area to the right, where the Xs tothe far right are unused spares and X between the remapped A6 and A8values indicates as defective redundancy column.

The Build In Self Test (BIST) mechanism for bad column addresses withbit information referred to above will now be described. This uses analgorithm to determine the bad column with bit information. A statemachine on the memory itself (not the controller) can execute theprocess for externally supplied test sequences and corresponding testdata. The flow chart of FIG. 17 will illustrate the steps. A majordifference from what would be a corresponding algorithm that did notneed to determine bit level errors, but only column level defects, isthat the bad column is NOT isolated right after each column test This isbecause the same column will be tested again. Another difference is thatthe error in the IO values (see FIG. 14) will be recorded for the eachbit.

FIG. 17 begins at 701 with starting the first of the tests (Column Test1) in the externally provided series. At 703, the expected data patternis compared with the data as written to and read back from the column,going through the columns and stopping at bad columns, as indicated bythe loop of 703 and 705. A circuit for executing this on the memory isshown in FIG. 18, where the read out data is compared with the expecteddata pattern to check the column error. As shown there, each of theexpected values (EXP<7:0>) is compared to the respected value for thecolumn as read out on the IO bus (YIO<7:0>). This yields thecorresponding match values for each bit, which are then combined toyield the BAD value as output. If BAD=1, at 705 the column address andmatch<7:0> value are recorded. This is preferable stored outside thearray for now as the array is still undergoing testing. For instance, ina multi-plane memory, this could stored in an unused plane. (Althoughthe other plane data latches may have unknown defects, multiple copiescan be used to guarantee the data integrity. For example, one set ofdata can be transferred to a set of data latches in the un-used planewith 4 copies of original data. If the chip has only one plane, separatedata latches into Left/Right or Top and Bottom partitions can be used.Only one partition of the bitlines is tested at a time, the otherpartition is used for temporary storage.)

To improve robustness, multiple copies of the column redundancyinformation (FIGS. 14A, 14B) are preferably saved along withcomplementary data (A and Ab copies). By saving the data in both the Aand Ab form, these can readily be compared to see if the data iscorrupted. On retrieving the data, the data and complementary data willbe compared, if they match, then the data will be validated to be gooddata. If the compare fails, then this copy of data will be discarded andnext copy of data will be fetched and compared until a good copy isfound. Another method of getting the correct data is that all the copiesof data are fetched and voted with the majority logic to determine theright data.

At 707, the next test is begun, with the expected data for this testagain compare with the read out data at 709. The stored result from 705is then fetched at 711 and compared with that from 709 for any addressmatches between the two. Address match can be done with XOR logic aswell, with an exemplary circuit for this is shown in FIG. 19, which cancompare the address of the new bad column with the bad column addressfound in the previous test to see these two address match or not. Thisis shown for the exemplary embodiment of 13 columns, where the Addr_newvalues are from 709 and the Addr_old are from 705. The results of thecomparisons (ADD_MATCH<12:0>) will generate logic signal SAME_ADDR,corresponding to 713. In case of an address match, the bit failureinformation can be updated and written back to where it is being held.The bad bit information update can be done with AND logic as in FIG. 20.The bad bit information is updated when the bad address matches thepreviously found bad column address. Some tests may have same bad bitaddress, some tests may turn out to have different bit address.

If there is no match at 713, a new entry is written back at 717. Both715 and 717 loop back to 709 and the process continues until the currenttest is done for all columns, after which the flow decides if there aremore tests at 719. If so, the flow loops back to 707 and if not, at 721the stored results from the series of test are fetched and the isolationlatches set for the columns found defective. The bad column informationwill also be written into the designated ROM block in the non-volatilememory. In some cases, the test flow could be broken into tests done atdifferent times. The test result can be stored in the ROM block forfirst few tests, and then the data will be read back from the ROM blockand continue with the subsequent tests following same test algorithm asdescribed above. Although the embodiment presented above is for aninitial sort based upon externally provided tests, alternate embodimentscould be performed to dynamically update the defect information, basedon tests executed, for example, by the controller or sophisticatedtester.

FIG. 21 is a schematic illustration for the on-chip management of thebad bits. A set of data to be written onto a wordline of array 801 isrepresented by addresses A0-A29, corresponding to regular,non-redundancy columns. Without taking any defects into account, thisset of data would be transferred to the appropriate data latches alongthe top and bottom of the array (as shown schematically by the arrows,corresponding to bus structures) and then written into the array.Considering now some defects, the bits at addresses A2, A6, A8, A13,A15, and A28 for this wordline and these columns are here taken asdefective. Based on the addresses for these bits, the data for thesebits are intercepted at a multiplexer MUX 821 and held in latches 815 inthe periphery and then programmed into the redundancy area 803, wherethe data along with its corresponding address is held. In this example,13 bits of address are used to specify the column to which the datacorresponds and 3 bits specify the bit within that column. The datavalues for these bad addresses can also be loaded into the data latchesalong the array or, if desired, they could be replaced with blank dataas the content of these addresses will be replaced with the data fromthe redundancy area during a read. In other embodiments, themultiplexing of values can be executed on the controller.

Considering the data in process further, this can be taken as the stepsof:

1) Data Shift into the Flash Memory and store the bad bytes in theperipheral latches;

2) The data will be packed into smaller data bytes by only extract thedata from bad bits;

3) Transfer the data to Column Redundancy columns.

The shifting can be executed by a set of clocked latches, examples ofwhich is shown in FIGS. 22A and 22B, allowing the data to be compactedfor storage in the redundancy area, as can be illustrated with FIGS.23-25. The latch structure of FIG. 22A would correspond to that used forthe pointer, as at the top of FIG. 24 or FIG. 26, and the latchstructure of FIG. 22B would correspond to that used for the data in anddata, as at the bottom of FIG. 24 or FIG. 26

FIG. 23 shows a stream of incoming data. This shows a series of byteswith the bad bits shown, the main part of the address (e.g., A2) showingthe column and the subscript indicating the bit in the byte along thatcolumn (e.g., the wordline of a NAND string) that is bad. Some byteshave multiple bad bits, others only a single bad bit. (Only the byteswith addresses corresponding to bad bits are shown.) Under each bit isthe bad bit information, “1” for good bits and “0” for bad. To save onstorage space, the bad bits can be compacted using data latches such asthat shown in FIGS. 22A and 22B: When the bad bit information is “1”,the latch will be selected and the bit data will flow out at the output.FIG. 24 shows a pointer based arrangement for column selection that canbe used to compact the data. (The use of pointers for column selectionis discussed further in U.S. Pat. No. 7,405,985 and references citedtherein.) Across the top is a series of latches (as in FIG. 22A)allowing the pointer to propagate one clock to toggle through all thelatches. The data latches at the bottom (as in FIG. 22B) receive theunpacked data and provide the packed version at DATA_OUT. FIG. 24functions similarly to FIG. 26 discussed below, which unpacks the data.This compacted data will then transferred to the data latches FIFO andformed into new bytes, as shown in FIG. 25, where the data fromnon-defective bits have been removed, leaving only that corresponding tothe defects.

The data out process will need undo the data in process and can be takenas the steps of:

-   -   1) After the sensing, the data in the column redundancy columns        are transferred out to the peripheral data latches;    -   2) The data will be re-shuffled back to byte form corresponding        to each bad columns, where the good bit data can be filled with        “1”;    -   3) The multiplexer mixes the data from the peripheral latches        back in when the user toggles out the data and the column        address maps to the bad columns.        The data out process (un-packing the data), may use many clock        cycles to finish the task. One arrangement for doing this can be        illustrated with FIGS. 26-28.

FIG. 26 shows a pointer based arrangement for column selection that canbe used to unpack the data. Across the top is a series of latchesallowing the pointer to propagate one clock to toggle through all thelatches. The data latches at the bottom receive the packed data atDATA_IN, which is FIFO register with single bit flow out at a time. Forthe data latches at bottom, only half of the clock signal inputs areused. In the middle is a set of select circuits having as inputs thepointer value and the bad bit information. An exemplary embodiment forthe select circuit is shown in FIG. 27. When the pointer is toggled to agiven latch and the bad_bit=0, then this address will be selected andthe data from the array will be latched into the latch. The pointer willthen continue going through all the latches until end of the latches isreached. FIG. 28 shows how the data in the redundant locations can beflowed out of a series of FIFO registers that have as inputs the CRDdata as inputs to compact the data. At the end of this process, the datafrom the redundancy area will be unpacked back to the form.

The on-chip implementation of the bad bit packing and un-packing may usea large number of registers, possibly increasing die size. One toimplement the process using a relatively small die area and a limitednumber of registers is to divide the bad bytes into several groups. Eachtime, a group of bad columns will be packed or unpacked with fixednumber registers to handle address and data information. The algorithmfor packing or un-packing can still be the same as described above. Forexample, if the memory have 40 bad bytes, it can process 10 bytes at atime and finish the bad byte processing in 4 groups. After instance ofpacking, the packed bytes can be put into the extended column area datalatches. After each instance of un-packing, the un-packed bits (orbytes) can be sorted back to their original data place. More details ofsuch an implementation, in a slightly different context, are presentedin U.S. patent application Ser. No. 12/414,935.

The techniques described above for the applications of bad column withbad bit information. The bit information will enhanced device yieldsince more bad columns with bad bits can be repaired with the fixednumber column redundancies typically available on a device. Besides thenormal operations, it also benefits the bad column management in thedevices incorporating an internal folding algorithm, such as thatdescribed in U.S. patent application Ser. No. 12/478,997.

The bad bits can be arranged in the column redundancy area as shown inthe example of FIG. 29. Three bytes, corresponding to three columns inthe main array, with address A, B, and C are shown. The individual bitsare identified by the IO values, corresponding to the bit on an IO busthat would transfer these bits for a corresponding set of wordlines. Thebad bits in the example are taken as A6, B6, B3 and C0 will be collectedto a column ColRD in the redundancy area. As discussed above, the goodbits in the bad columns can stay there and get programmed, even thoughthe bad column isolation latch will be set to skip the programcompletion detections.

The reason to set the bad column isolation latch is that some failurescould cause detection fail if the detection is done collectively andsimultaneously, but these failure bits should not be counted as they arealready repaired by the redundancy. This could lead to overly strictcriteria to pass program (or erase) and make the operations return withfailed status. For example, if there are 20 bad column repaired by theredundancy columns, these 20 bad columns will cause 40 bits failures. Ifthe program pseudo-pass criteria is set to be 40, then there will be 0failures allowed for the whole page program. If the program pseudo-passcriteria is set to be less than 40, the page program will always fail.When such situations occur, the status will not reflect the realsituation as to whether the write operation has succeeded or not. Inorder to make sure that the program status reflect the real programsituation, the bad columns should be masked out or isolated. If the badbits are counted serially by toggling the data out one byte (or a word)at a time, then the isolation latch is not necessary.

This sort of bit level management can be particularly advantageous forincorporating an internal folding, as that described in U.S. patentapplication Ser. No. 12/478,997. Briefly, data is initially written to amemory in binary form, folded into a multi-state format in the memorylatches, and then rewritten back into the non-volatile memory. To take a3-bit per cell example, three pages would initially be written ontothree physical pages in binary form and then rewritten in 3-bit per cellformat onto a single physical wordline. In the case of a bad column,this defect will need to be reflected in the columns with which it isfolded, leading to a corresponding increase in number of redundantcolumns used.

This process can be illustrated with FIG. 30. In FIG. 30, the XDL latchis the data latch through which an input-output circuit communicateswith the data buses and ADL, BDL, and CDL correspond to the data latchesfor holding each of the bits for a multi-bit (here 3-bit) programmingoperation. In the folding operation, three separate wordlines with datain a binary format are read in the XDL latches. Here, A, B, C, refer tothe wordlines (or physical page) and the numbers (0-4607) to the columnsas these bits are stored on three separate, or upper (U), middle (M) andlower (L) wordlines. The bytes are then rearranged from the original 3pages of data in XDL to into the data latches ADL, BDL and CDL. Thecontent of the ADL, BDL, and CDL latches are then all programmed into asingle physical page. (This is again described in more detail in U.S.patent application Ser. No. 12/478,997, although the exemplary foldingthere differs some.)

Because of this, a bad column will need to be reflected in the othercolumns with which it is folded. Consequently, in an N-bit per cellfolding process, each bad column may be magnified by a factor of N,which could quickly exhaust the available number of redundant columns.Because of this, the use of bit information for bad column can beparticularly advantageous in system that use such folding. Even thoughthe folding process will create more failed bits during the process offolding, the bad bits management will reduce the impact of wasting toomany redundancy columns because of folding.

Although the various aspects of the present invention have beendescribed with respect to certain embodiments, it is understood that theinvention is entitled to protection within the full scope of theappended claims.

1. A non-volatile memory circuit, comprising: an array of non-volatilememory cells formed along columns of multiple bits, the columnsincluding a plurality of regular columns and one or more redundancycolumns, and a plurality of latches, each corresponding to one of theregular columns and having a bit whose value indicates if thecorresponding column is defective, the memory circuit storing a columnredundancy data table whose contents indicate for each redundancy columnwhether the redundancy column is being used and, for redundancy columnsthat are being used, a defective regular column to which it correspondsand the bits therein which are defective, wherein the memory circuitstores data corresponding to the defective bits of defective regularcolumns in the redundancy column portion.
 2. The non-volatile memorycircuit of claim 1, wherein each column corresponds to a byte of data.3. The non-volatile memory circuit of claim 1, wherein the content ofthe column redundancy data table further indicate for each redundancycolumn whether the redundancy column is defective.
 4. The non-volatilememory circuit of claim 1, wherein the content of the column redundancydata table and the value of the plurality of latches are determined in atest process.
 5. The non-volatile memory circuit of claim 1, wherein thememory circuit stores multiple copies of the column redundancy datatable.
 6. The non-volatile memory circuit of claim 1, wherein the memorycircuit further stores the column redundancy data table in complementaryform.
 7. The non-volatile memory circuit of claim 1, wherein regularcolumns whose corresponding latch value indicates the regular column isdefective and where the column redundancy data table indicates that lessthan all of the bits therein are defective are used to store valid datain the non-defective bits thereof.
 8. The non-volatile memory circuit ofclaim 1, further comprising: data latch circuitry connectable to theredundancy columns and responsive to the column redundancy data tablecontents, whereby the data corresponding to the defective bits ofdefective regular columns are packed into a compacted form for storagein the redundancy column portion in a write operation and unpacked in aread operation.
 9. The non-volatile memory circuit of claim 8, whereinthe data corresponding to the defective bits of defective regularcolumns are packed and unpacked in multi-bit groups.
 10. Thenon-volatile memory circuit of claim 1, wherein the contents of thecolumn redundancy data table further includes a failure mode for thedefective regular columns.
 11. A method of operating a non-volatilememory circuit, the memory circuit including an array of non-volatilememory cells formed along columns of multiple bits and having a latchassociated with each of the columns whose value indicates if thecorresponding column has a defect, the method comprising, performing awrite operation to concurrently program a plurality of memory cells on acorresponding plurality of columns, including one or more columns havingan associated latch whose value indicates the corresponding column has adefect; determining the number of the plurality of concurrentlyprogrammed memory cells that were not successfully programmed in thewrite operation, wherein the columns whose latch values indicate thecolumn has a defect are not counted in the determining; and determiningwhether the number of cells that were not successfully been programmedduring the write operation is acceptable.
 12. The method of claim 11,wherein the write operation includes a plurality of alternating pulseand verify operations for all of the cells on which the write operationis being performed, with cells that verify as correctly programmed arelocked out from further pulses.
 13. The method of claim 11, wherein thenumber of cells acceptable as not successfully programmed is apredetermined value based on the error correction code capabilities ofthe memory system of which the memory circuit is a part.
 14. The methodof claim 11, further comprising: prior to said programming operation,determining for each of the one or more columns having an associatedlatch whose value indicates the corresponding column has a defect if thecell to be programmed in the corresponding column is defective; and inresponse to determining that the cell to be programmed in thecorresponding column is defective, saving the data to programmed thereinto a redundancy area of the memory array.
 15. The method of claim 14,wherein said determining is based upon a column redundancy data tablestored the non-volatile memory circuit whose contents indicate whichbits are defective for columns whose associated latch whose valueindicates the corresponding column has a defect.
 16. A method ofoperating a non-volatile memory circuit having an array of non-volatilememory cells formed along columns of multiple bits, the columnsincluding a plurality of regular columns and one or more redundancycolumns, the method comprising: performing a plurality of column testoperations to determine which columns are defective and the individualbits therein which are defective, each of the column tests including:writing and reading back an externally supplied data pattern to thecolumns; and comparing the externally supplied data pattern as read backwith an expected data pattern, wherein said column test operation areperformed by circuitry on the memory circuit and each of the columntests uses a different data pattern; recording addresses of any of theregular columns determined defective and the individual bits thereinwhich are determined defective in a column redundancy data table storedon the memory circuit; and for any of the regular columns determineddefective, setting a latch associated therewith to a value indicatingthat the associated column is defective.
 17. The method of claim 16,further comprising: for any of the redundant columns determineddefective, setting corresponding flag values in the column redundancydata table.
 18. The method of claim 16, wherein the column testoperations are executed by a state machine on the memory circuit. 19.The method of claim 16, wherein the recording addresses of any of theregular columns determined defective and the individual bits thereinwhich are determined defective, includes temporarily storing the resultsof said comparing on another array of the memory circuit.
 20. The methodof claim 16, wherein the recording addresses of any of the regularcolumns determined defective and the individual bits therein which aredetermined defective, includes storing multiple copies of the columnredundancy data table on the memory circuit.
 21. The method of claim 16,wherein the column test operations further include: determining afailure mode for the regular columns determined defective; and recordingthe failure mode in the column redundancy data table for the regularcolumns determined defective.
 22. A method of operating a non-volatilememory circuit having an array of non-volatile memory cells formed alongcolumns of multiple bits, the columns including a plurality of regularcolumns and one or more redundancy columns, the method comprising:storing on the memory circuit a column redundancy data table whosecontents indicate for each redundancy column whether the redundancycolumn is being used and, for redundancy columns that are being used, adefective regular column to which it corresponds and the bits thereinwhich are defective; receiving a set of data to program into the memoryarray; determining the elements of the set of data assigned to beprogrammed to defective bits of defective regular columns based upon thecolumn redundancy circuit data table; storing the elements of the set ofdata determined to be assigned to be programmed to defective bits ofdefective columns in peripheral latch circuits on the memory circuit;storing the set of data into programming latches for the memory array;performing a programming operation into the regular columns of thememory array from the programming latches; and programming the elementsof the data set stored in the peripheral latches into the redundancycolumns.
 23. The method of claim 22, further comprising: prior toprogramming the elements of the data set stored in the peripherallatches into the redundancy columns, performing a packing operation onthe memory circuit for the elements of the set of data determined to beassigned to be programmed to defective bits of defective columns,whereby elements of data assigned to be programmed to more than oneregular column are programmed into a single redundant column.
 24. Themethod of claim 23, wherein said packing operation includes a pluralityof sub-operations, each performing a packing operation on a subset ofthe set of data determined to be assigned to be programmed to defectivebits of defective columns.
 25. The method of claim 22, wherein thecontents of the column redundancy data table further includes a failuremode for the defective regular columns.